1. Field of the Invention
The present invention relates to lasers, and in particular to optimization of laser bar orientation for nonpolar or semipolar (Ga,Al,In,B)N diode lasers.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within parentheses, e.g., (Ref. X). A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The usefulness of gallium nitride (GaN) and alloys of (Ga,Al,In,B)N (i.e., “Group-III nitride,” also referred to as “III-nitride” or just “nitride”) has been well established for fabrication of visible and ultraviolet optoelectronic devices and high power electronic devices. Current state-of-the-art nitride thin films, heterostructures, and devices are grown along the [0001] axis of the würtzite nitride crystal structure, which is shown in FIG. 1A. The total polarization of such films includes both spontaneous and piezoelectric polarization contributions, both of which originate from the single polar [0001] axis of the würtzite nitride crystal structure. When nitride heterostructures are grown pseudomorphically, polarization discontinuities are formed at surfaces and interfaces within the crystal. These discontinuities lead to the accumulation or depletion of carriers at surfaces and interfaces, which in turn produce electric fields. Since the alignment of these polarization-induced electric fields coincides with the typical [0001] growth direction of nitride thin films and heterostructures, these fields have the effect of “tilting” the energy bands of nitride devices.
In c-plane würtzite nitride quantum wells, the “tilted” energy bands (conduction band Ec and valence band Ev) spatially separate the electron wavefunction and hole wavefunction, as illustrated in FIG. 1B. This spatial charge separation reduces the oscillator strength of radiative transitions and red-shifts the emission wavelength. These effects are manifestations of the quantum confined Stark effect (QCSE) and have been thoroughly analyzed for nitride quantum wells (Refs. 1-4). Additionally, the large polarization-induced electric fields can be partially screened by dopants and injected carriers (Refs. 5-6), making the emission characteristics difficult to engineer accurately.
Furthermore, it has been theoretically predicted that pseudomorphic biaxial strain has little effect on reducing the effective hole mass in c-plane würtzite nitride quantum wells (Ref. 7). This is in stark contrast to typical III-V zinc-blende InP-based and GaAs-based quantum wells, where anisotropic strain-induced splitting of the heavy hole and light hole bands leads to a significant reduction in the effective hole mass. A reduction in the effective hole mass leads to a substantial increase in the quasi-Fermi level separation for any given carrier density in typical III-V zinc-blende InP-based and GaAs-based quantum wells. As a direct consequence of this increase in quasi-Fermi level separation, much smaller carrier densities are needed to generate optical gain (Ref. 8). However, in the case of the würtzite nitride crystal structure, the hexagonal symmetry and small spin-orbit coupling of the nitrogen atoms in biaxially strained c-plane nitride quantum wells produces negligible splitting of the heavy hole and light hole bands (Ref. 7). Thus, the effective hole mass remains much larger than the effective electron mass in biaxially strained c-plane nitride quantum wells, and very high current densities are needed to generate optical gain in c-plane nitride diode lasers.
One approach to eliminating polarization effects and decreasing the effective hole mass in nitride devices is to grow the devices on nonpolar planes of the crystal. These include the {11-20} planes, known collectively as a-planes, and the {10-10} planes, known collectively as m-planes. Such planes contain equal numbers of gallium and nitrogen atoms per plane and are charge-neutral. Subsequent nonpolar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction. Moreover, unlike strained c-plane InxGa1-xN quantum wells, it has been theoretically predicted that strained nonpolar InxGa1-xN quantum wells should exhibit anisotropic splitting of the heavy hole and light hole bands, which should lead to a reduction in the effective hole mass for such structures (Ref. 9). Self-consistent calculations of many-body optical gain for compressively strained InxGa1-xN quantum wells suggest that the peak gain is very sensitive to effective hole mass and net quantum well polarization and that peak gain should increase dramatically as the angle between a general growth orientation and the c-axis increases, reaching a maximum for growth orientations perpendicular to the c-axis (i.e., on nonpolar planes) (Refs. 10-11).
Commercial c-plane nitride LEDs do not exhibit any degree of polarization anisotropy in their electroluminescence. Nonpolar m-plane nitride LEDs, on the other hand, have demonstrated strong polarization anisotropy in their electroluminescence along the [11-20] axis (Refs. 12-14). This polarization can be attributed to anisotropic strain-induced splitting of the heavy hole and light hole bands in compressively strained m-plane InxGa1-xN quantum wells, which leads to significant disparities in the [11-20] and [0001] polarized optical matrix elements. Likewise, it can be expected that the optical emission from m-plane nitride diode lasers should show similar polarization anisotropy.